This invention relates to a unit fuel injector for a diesel engine, and more particularly to an electromagnetic injection control valve in the unit fuel injector.
In diesel engines each cylinder is equipped with a fuel injection nozzle to which pressurized liquid fuel is supplied from a fuel injection pump.
In a so-called unit fuel injector such as the one shown in U.S. Pat. No. 4,129,253, the fuel injection nozzle and an injection pump are united into a single compact device together with an electromagnetic control valve to permit the injection nozzle to make fuel injection at suitable timing. Major advantages of such a unit fuel injector are attributed to the omission of a relatively long fuel injection pipe for connection of the injection nozzle to the pump. Naturally the injection time lag is reduced, and the injection pressure can be increased with a favorable effect on the atomization of fuel because the omission of a long injection pipe means a considerable decrease in the volume of fuel to be pressurized. Besides, the rate of fuel injection can be augmented and dribbling of fuel after the termination of injection can be lessened.
In a unit fuel injector, fuel at a relatively low pressure is supplied from a fuel tank by means of a fuel pump to a pressure chamber in the injection pump portion where the fuel is intensely pressurized. The injection nozzle portion of the unit injector has a needle valve to normally close the spray-holes, and when the pressure of fuel transmitted from the injection pump portion reaches a predetermined valve-opening pressure the needle valve lifts to open the spray-holes. The electromagnetic injection control valve is a normally open valve provided to a fuel passage connecting the pressure chamber in the injection pump portion to the fuel tank. Therefore, the fuel pressure in the pressure chamber leaks out through this control valve and, hence, does not reach the aforementioned valve-opening pressure so long as the control valve remains in the open position. The electromagnetic control valve closes in response to an electrical current pulse signal supplied from a fuel injection control circuit. Then the pressure of fuel in the pressure chamber begins to effectively increase and soon exceeds the valve-opening pressure in the nozzle portion to cause injection of fuel. Upon termination of the supply of the pulse signal, the electromagnetic control valve returns to its open position by the force of a spring to result in lowering of the fuel pressure in the injection pump portion and nozzle portion and then termination of fuel injection.
At the end of fuel injection, rapid lowering of the injection pressure is desired with a view to realizing clean cut-off of injection. In a unit fuel injector the rate of lowering of the injection pressure depends on the effective area of a leak orifice defined in the control valve between a valve seat and a tip portion of a needle valve, and the effective area of this orifice is limited by the maximum amount of the lift of the valve. One method of increasing the leak orifice area is to diametrically enlarge the valve seat. However, enlargement of the valve seat needs to be accompanied by augmentation of the electromagnetic force for seating of the valve. For example, where the injection pressure is about 1000 atm the seating of the valve requires a force of about 30 kgf or more even though the valve seat diameter is as small as about 2 mm. A substantial augmentation of the electromagnetic force to enlarge the valve seat naturally results in enlargement of the size of the electromagnetic control valve and, besides, raises difficulty in realizing quick responsiveness of the electromagnetic control valve required for application of the unit fuel injector to a compact and high-speed diesel engine. Another method of increasing the leak orifice area is to increase the maximum amount of the valve lift. This means an increase in the gap between the armature and core of the electromagnetic device. Since the electromagnetic force required for attraction of the armature is proportional to the square of the gap width, when the maximum amount of the valve lift is increased the electromagnetic force must be augmented in proportion to the square of the increased valve lift. This is unfavorable for the reasons explained above.